The present studies further characterize natural killer (NK) cells in the rat. The results demonstrate substantial differences between the cells which mediate NK activity (LGL) and normal rat T cells. These differences include antigen expression, ability to recirculate, precursor turn-over rate, anatomic and organ distribution, expression of the T cell receptor and production of interferon (IFN). These differences in a number of basic biological properties suggest that LGL and T cells represent separate populations of lymphocytes which share some common antigenic and functional properties. The present data is consistent with the hypothesis that LGL represent an entirely separate, third lineage of lymphocytes, distinct from both T and B cells.